NMR imaging method and apparatus

ABSTRACT

An NMR imaging method and apparatus according to the present invention which enables a reduction in the scanning time without lowering the contrast resolving power and the spatial resolving power is arranged such that data which belongs to a relatively low spatial frequency region is collected by an ordinary scanning method, while data which belongs to a relatively high spatial frequency region is collected by a scanning method which enables data to be collected at a relatively high frequency, and reconstruction of an image is effected on the basis of data synthesized from data collected by these two methods.

DESCRIPTION

1. Technical Field

The present invention relates to improvements in an NMR imaging methodand apparatus for obtaining a cross-sectional image of an object ofinspection by utilizing nuclear magnetic resonance. More particularly,the present invention pertains to an NMR imaging method and apparatuswherein the scanning time required to collect NMR signals by the Fouriertransformation method is reduced.

2. Background Art

An NMR imaging apparatus has a magnet section including a staticmagnetic field coil for producing a uniform static magnetic field H₀ anda gradient magnetic field coil for producing a magnetic field whichextends in the same direction as the static magnetic field H₀ and whichhas a linear gradient in each of the x, y and z directions, atransmission and reception section which is arranged to applyradio-frequency pulses (radio-frequency electromagnetic wave) to anobject of inspection placed within the magnetic field formed by themagnet section and to detect an NMR signal from the object, a controland image processing section which is arranged to control the operationof the transmission and reception section and that of the magnet sectionand to process detected data to thereby display an image, and othersections or members.

In such an NMR imaging apparatus, the transmission and reception sectionis under the control of the control and image processing section so asto output radio-frequency pulses in a sequence in accordance with thesaturation recovery (SR) method or the inversion recovery (IR) methodand on the basis of the multislice method. The control and imageprocessing section effects collection of data on the basis of theFourier transformation method in order to reconstruct an image.

In the above-described radio-frequency pulse sequence, the recovery ofexcited spins depends on a natural relaxation process. Accordingly, theproportion of the data observation time with respect to the scanningtime is low and the S/N per time is low. Further, since the spinexcitation interval is determined in view of the recovery of spins, thescanning time is correspondingly long, so that the rate of incidence ofartifacts due to the movement of the object's body is high. On the otherhand, the multislice method enables a plurality of images to be obtainedwithin the same scanning time. However, there is no change in the entirescanning time and, therefore, this method is helpless against artifactsdue to the movement of the object's body.

To shorten the scanning time in the SR or IR method, it is necessary toreduce the number of times of phase encoding (spin warp), and this leadsto a lowering in the spatial resolving power of the NMR image.

One example of methods which have heretofore been invented in order tosolve these problems is the FR (Fast Recovery) method disclosed, forexample, in Japanese Patent Laid-Open Nos. 231438/1984 and 29684/1985.The feature of this method resides in that a radio-frequency pulsesequence is applied without awaiting spins to recover to a thermalequilibrium state so as to forcedly direct the magnetization M (the sumtotal of spins) toward the z-axis direction. More specifically, thesequence according to the FR method is so designed that the scanningspeed is given priority and scanning is thereby completed within a shortperiod time. Thus, it is possible to shorten the scanning time itself.

The conventional NMR imaging apparatus suffers, however, from thefollowing problems. Since the recovery of spins to a thermal equilibriumstate is forcedly effected, the contrast of an NMR image whichoriginally strongly depends on the relaxation time is deteriorated. Morespecifically, the prior art has the problem that as the scanning time isreduced (as the speed of scanning is increased), the contrast of theresultant NMR image is degraded.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an NMR imagingmethod and apparatus which is so designed that the scanning time isreduced without any lowering in the quality of an image, such as thecontrast resolving power and the spatial resolving power.

To this end, the present invention provides an NMR imaging method andapparatus wherein data which belongs to a relatively low spatialfrequency region is collected by an ordinary scanning method, while datawhich belongs to a relatively high spatial frequency region is collectedby a scanning method which enables data to be collected at a relativelyhigh frequency, and reconstruction of an image is effected on the basisof data synthesized from data collected by these two methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of the present invention;and

FIGS. 2 to 4 are views employed to describe the operation of theembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described hereinunder in detail withreference to the drawings.

FIG. 1 is a block diagram of one embodiment of the present invention. Amagnet assembly 1 has a space portion therein for receiving an object ofinspection. The following various coils are disposed in such a manner asto surround this space portion: namely, a static magnetic field coil forapplying a predetermined static magnetic field to the object; x-, y- andz-gradient magnetic field coils for generating gradient magnetic fields;an RF transmission coil for applying radio-frequency pulses to theobject in order to excite nuclear spins within the object; a receptioncoil for detecting an NMR signal from the object; etc. (not shown). Thestatic magnetic field coil, the gradient magnetic field coil, the RFtransmission coil and the reception coil are connected to a mainmagnetic field power supply 2, a gradient magnetic field driving circuit3, an RF power amplifier 4 and a pre-amplifier 5, respectively. Asequence memory circuit 10 actuates a gate modulating circuit 6 for eachview in accordance with a command from a computer 13 and applies aradio-frequency pulse signal based on the SR or FR method to the RFtransmission coil through the RF power amplifier 4 in accordance witheach of the sequences stored in advance. The sequence memory circuit 10further actuates the gradient magnetic field driving circuit 3, the gatemodulating circuit 6 and an A/D converter 11 in accordance with asequence based on the Fourier transformation method. A phase detector 8uses the output of the RF oscillating circuit 7 as a reference signal tocarry out phase detection of an NMR signal which is detected by thereception coil and delivered through the pre-amplifier 5 and applies theNMR signal to the A/D converter 11. The A/D converter 11 subjects theNMR signal obtained through the phase detector 8 to analog-to-digitalconversion and applies the converted NMR signal to the computer 13. Thecomputer 13 exchanges information with a control console 12, switchesover the operation of the sequence memory circuit 10 from one to anotherand also rewrites the memory in the sequence memory circuit 10 in orderto realize various scanning sequences, performs calculation forreconstructing the distribution of information concerning the resonanceenergy into an image by the use of data supplied from the A/D converter11, and outputs data concerning the reconstructed image to a display 9.

It is also possible to store a sequence according to each of the SR andFR methods in the computer 13 and rewrite the memory in the sequencememory circuit 10 according to need.

The operation of the above-described arrangement will next be explained.

Generally speaking, the contrast resolving power gives rise to a problemat a relatively low spatial frequency, that is, a portion having arelatively wide area. Accordingly, the sequence memory circuit 10actuates, under the control of the computer 13, each circuit and thelike with a sequence signal based on the SR method for a view of smallamount of warp corresponding to a low spatial frequency, therebyeffecting the operation shown in FIG. 2. The operation shown in FIG. 2is known as the spin warp method, although it will be detailed later. Atthis time, the contrast at a target imaging position can be increased byselecting predetermined values for scanning parameters.

On the other hand, for a view other than the abovedescribed view whichhas a large amount of warp corresponding to a high spatial frequency,i.e., a portion for which the contrast resolving power gives rise to noserious problem, the sequence memory circuit 10 actuates each circuitand the like with a sequence signal based on the FR method, therebyeffecting the operation shown in FIG. 3. The operation shown in FIG. 3is known a the FR method, althogh it will be detailed later. Thus,collection of data in a view of large amount of warp can be effectedwithin a short period of time.

Switching of the SR method and the FR method from one to the other whichis effected on the basis of the amount of warp is carried out under thecontrol of the computer 13 with some overlapping portions provided.Accordingly, there is no fear of collection of data being madediscontinuous by switching of the two methods. Data collected by each ofthe sequence signals is weighted in accordance with the spatialfrequency region as shown in FIG. 4, that is, subjected to filtering inaccordance with the spatial frequency, and then synthesized. With thesynthesized data, reconstruction of an image is carried out by the samemethod as in the prior art. It should be noted that the amount of phaseencoding in FIG. 4 means the amount of warp which corresponds to thespatial frequency of data to be collected.

Thus, it is possible to reconstruct an image without lowering the imagecontrast resolving power, the spatial resolving power and the like byselecting a radio-frequency pulse generating sequence in accordance withspatial frequency characteristics in scanning data. Since a portion ofhigh spatial frequency is subjected to data collection by the FR methodwith which scanning is effected at high speed, even if the SR methodtakes somewhat long scanning time, it is possible to realize a reductionin the overall scanning time. If the speed of the FR method is assumedto be, for example, 10 times that of the SR method, the degree to whichthe speed of the above-described operation is increased is as follows.When 2/10 of the whole views including the overlapping portions isscanned by the SR method and 9/10 is scanned by the FR method, theoverall scanning time is reduced to 29% of that in the case of scanningthe whole views by the SR method as given by the following equation:

    2/10+9/10×1/10=29/100=29%

The operations shown in FIGS. 2 and 3 will next be explained in detail.

The sequence shown in FIG. 2 . . . . Under a uniform static magneticfield H₀ produced by the main magnetic field power supply 2, thesequence memory circuit 10 actuates the gradient magnetic field drivingcircuit 3 and the gate modulating circuit 6 so as to generate each ofthe gradient magnetic fields and radio frequency pulses. The sequencememory circuit 10 further actuates the A/D converter 11 so as to convertan NMR signal detected by the phase detector 8 into a digital signal andinputs it to the computer 13. More specifically, the sequence memorycircuit 10 applies a 90° pulse (see FIG. 2(a)) while applying a slicegradient G_(z) (see FIG. 2(b)). In consequence, spins within a specificslice plane of an object of inspection alone are selectively excited.Then, the following gradients are applied: namely a rephase gradientG_(z) (see FIG. 2(b)) for recovering the phase shift of spins caused atthe time of slicing; a dephase gradient G_(x) (see FIG. 2(d)) forgenerating an echo signal later; and a warp gradient G_(y) (see FIG.2(c)). Thereafter, the application of all the gradients is cut off, anda 180° pulse is then applied. In consequnce, the spins are inverted, anda spin echo signal is then obtained (see FIG. 2(e)) by the subsequentapplication of a read-out gradient G_(x) (see FIG. 2(d)). This echosignal corresponds to one of the lines obtained by subjecting to thetwo-dimensional Fourier transformation the distribution of intensitiesof signals from the spins in the object. The selection of lines iseffected by means of the product of the amount of application of they-gradient, i.e., the magnitude of the y-gradient magnetic field g_(y),and the application time T_(w). Thereafter, the above-described sequenceis repeated with the gradient G_(y) varied, thus data required toreconstruct an image being collected.

The sequence shown in FIG. 3 . . . . The operation of each circuitduring the period which begins at the time t₁ and which ends at the timet₂ is basically the same as that shown in FIG. 2. A 90° pulse is furtherapplied at the time t₂ at which the spin echo signal reaches a maximum.In consequence, the magnetization M (the sum total of spins) is forcedlydirected toward the z-axis direction without awaiting the spins torecover to a thermal equilibrium state, and the magnetization Mcoincides with the z-axis when the time t_(d) (sufficiently shorter thanthe longitudinal relaxation time T₁) has elapased after t₂. Thus, thefirst sequence is completed and a similar sequence is repeatedthereafter successively with the amount of warp varied. The applicationof a 90° pulse at the time t₂ as described above enables a reduction inthe data collecting time per view and hence a reduction in the overallscanning time.

It should be noted that the present invention is not necessarily limitedto the above-described embodiment. For example, the IR and FR methodsmay be combined together. It is also possible to employ a sequence basedon the SR method and arrange the scanning parameters, e.g., therepetition time, so as to vary with views. More specifically, thearrangement may be such that, for a portion of large amount of phaseencoding which contributes less to the contrast resolving power and S/N,the repetition time TR is shortened to increase the scanning speed,whereas, for a portion of small amount of phase encoding, the repetitiontime TR is returned to the previous value.

Although the present invention has been described above by way of thebest mode for carrying out it, it will be easy for those who haveordinary knowledge of the technical field to which the present inventionbelongs to make various modifications without departing from the scopeof the following claims.

I claim:
 1. In an NMR imaging method wherein a static magnetic field,gradient magnetic fields in x-, y- and z- directions and aradio-frequency electromagnetic wave are applied to an object ofinspection, an NMR signal from the object is measured by the Fouriertransformation method with the amount of warp of spins varied for eachview, and a cross-sectional image of the object is reconstructed on thebasis of the NMR signal,the improvement which comprises measuring theNMR signal with the SR or IR method for low spatial frequency; measuringthe NMR signal with the FR method so as to be shorter in time than theSR or IR method for high spatial frequency; and reconstructing across-sectional view of the object on the basis of the data synthesizedfrom the data obtained by these measurements.
 2. An NMR imaging methodaccording to claim 1, wherein the frequency range of measurement of theNMR signal with the SR or IR method for low spatial frequency and thefrequency range of measurement of the NMR signal with said FR method forhigh spatial frequency overlap each other at a predetermined range ofspatial frequencies.
 3. An NMR imaging method according to claim 2,wherein, with respect to data obtained by the SR or IR method and dataobtained by the FR method, portion of these data which overlap eachother are weighted for the same spin warp amounts.
 4. An NMR imagingmethod according to claim 1, wherein the measurement of the NMR signalis based on the SR method.
 5. An NMR imaging method according to claim1, wherein the measurement of the NMR signal is based on the IR method.6. In an NMR imaging apparatus having means (1) and (2) for applying astatic magnetic field to an object of inspection, means (1) and (3) forapplying gradient magnetic fields in x-, y- and z- directions on thebasis of the Fourier transformation method to the object, means (1),(4), (6) and (7) for applying a radio-frequency electromagnetic wave tothe object; means (1), (5), (8) and (11) for measuring an NMR signalfrom the object, and means (13) for reconstructing a cross-sectionalimage of the object on the basis of the measured NMR signal,theimprovement which comprises: means (10) for measuring the NMR signalwith the SR or IR method for low spatial frequency; means (10) formeasuring the NMR signal with the FR method so as to be shorter in timethan the SR or IR method for high spatial frequency; and means (13) forsynthesizing the data obtained by these measuring means andreconstructing a cross-sectional view of the object on the basis of thesynthesized data.
 7. An NMR imaging apparatus according to claim 6,wherein said means for measuring the NMR signal comprises measuringmeans based on the SR method.
 8. An NMR imaging apparatus according toclaim 6, wherein said means for measuring the NMR signal comprisesmeasuring means based on the IR method.